This invention relates to a method for controlling writing timings of picture signals to be stored in a memory, picked up by picture scanning means, for use in a picture reproducing machine such as a color scanner and a color facsimile.
A picture reproducing machine such as a color scanner for plate-making, or the like, having a buffer memory between a picture scanning means and a reproduction recording means, which controls reproduction scales and layout positions of reproduction pictures, has been proposed. In this case, in order to minimize the capacity required of such a buffer memory, picture and recording cylinders are rotated at the same speed, and each writing or reading operation of a picture signal is carried out one by one in a sampling interval, but, when the reproduction scale is changed at a certain value, several writing or reading operations are conducted in the sampling interval.
This sampling interval corresponds to the interval of picture elements, which determines the analytical power of the picture scanning means and the reproduction recording means. In the color scanner for plate-making, the analytical power required is very high, and thus the sampling interval is quite short.
The picture and the recording cylinders are controlled accurately so as to be revolved at the same speed. However, a very small amount of inequality in the rotation of the cylinders due to various mechanical errors is inevitable. Accordingly, the very small amount of relative speed variations of the cylinders do occur, and, in practice, it is impossible to adjust every starting position per one rotation of the scannings with an accuracy more than the analytical power of the picture signals.
Hence, the writing and the reading of the picture signal in a single sampling interval may be performed by using a clock pulse which is synchronous with either the picture scanning means or the reproduction recording means, but by using this clock pulse, as mentioned above, when the relative speed variations of the cylinders occur, the reproduction pictures are deformed largely rather than depending on the accuracy of each picture scanning or reproduction recording means.
In order to avoid the deformation of the reproduction picture, the writing should be carried out by a clock pulse which is synchronous with the picture scanning means and the reading should be carried out by a clock pulse which is synchronous with the reproduction recording means. However, the accesses of the writing and the reading in the memory are not always carried out in the same time. Accordingly, by the partial relative speed variation of the cylinders or the disorder of the starting positions of the scannings the phase of the clock pulses for writing and reading are shifted irregularly, and thus the timings of the writing and the reading clock pulses are overlapped, with the result of the deformation of the reproduction picture by the lack of the picture element, and so forth.
Further, the phases of the writing and the reading clock pulses are in proportion to the rotation speeds of the picture and the recording cylinders, and, if the speeds of the two cylinders synchronized two clock pulses having the same phase are obtained. Hence, the phase variations of the two clock pulses by the rotation inequalities, and the like, causes a beat.
This kind of beat is a zero beat, or the like, having a long frequency, and therefore, if the timings of the two clock pulses overlap each other, such a condition will continue for a relatively long time, resulting in a large scale of deformation of the reproduction picture.
In a conventional layout scanner, in which a plurality of original pictures are reproduced on a recording film according to the desired layout, a plurality of picture scanning means and a reproduction recording means are arranged in parallel, and a buffer memory is connected therebetween, wherein the picture signals obtained by the picture scanning means are stored into the memory, and then by reading out the picture signals consecutively of the memory the reproduction pictures are reproduced in the desired layout positions by the reproduction recording means. In this embodiment, since the plurality of picture scanning means are driven in parallel, there is less time to control the writing timings of the picture signals and thus it is rather difficult to do it.